1. Field of the Invention
The present invention generally relates to nuclear medicine, and systems for obtaining nuclear medicine images of a patient's body organs of interest. In particular, the present invention relates to systems and methods for obtaining nuclear medicine images by detecting coincident events resulting from positron annihilation.
2. Description of the Background Art
Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images which show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions which emanate from the body and are captured by a scintillation crystal, with which the photons interact to produce flashes of light or “events.” Events are detected by an array of photodetectors, such as photomultiplier tubes, and their spatial locations or positions are calculated and stored. In this way, an image of the organ or tissue under study is created from detection of the distribution of the radioisotopes in the body.
One particular nuclear medicine imaging technique is known as Positron Emission Tomography, or PET. PET is used to produce a three-dimensional image for diagnosing the biochemistry or physiology in a specific organ, tumor or other metabolically active site.
In PET, events are detected from the decay or annihilation of a positron. As shown in FIG. 1, when a positron 100 is annihilated within a subject 10, two 511 KeV gamma rays 101a and 101b are simultaneously produced which travel in approximately opposite directions. Two scintillation detectors 12a and 12b are positioned on opposite sides of the patient 10 such that each detector will produce an electrical pulse in response to the interaction of the gamma rays 101a and 101b with a scintillation crystal 14. In order to distinguish the detected positron annihilation events from background radiation or random events, the events must be coincident in each detector in order to be counted as “true” events.
True events result when the two 511 KeV photons from a single positron annihilation travel directly to opposite detectors and are absorbed by the respective scintillation crystals. A second type of event occurs when one or both of the 511 KeV photons is deflected from its original trajectory, either in the patient or in the crystal. This is known as Compton interaction or scatter. Because of their high energy, the percentage of 511 KeV photons which interact with the scintillation crystal without scatter is relatively small. Most of the 511 KeV photons pass through the crystal without interaction, while over half of the 511 KeV photons that do interact with the crystal undergo Compton scatter. Consequently, simply increasing the sensitivity of the crystal will result in the detection of an increased number of invalid events. As such, it is desirable in coincidence detection imaging to improve the accuracy of acquired images, by increasing the number of true events detected.
It is known in the prior art to use Compton events in positron imaging, see U.S. Pat. No. 3,955,088 to Muehllehner et al. Compton events may be usefully added to the stored distribution because the location of the origin of the event in the object under study can be calculated from the point of interaction of the Compton event in the crystal.
However, in the prior art as exemplified by the '088 patent, a pair of events is passed on for processing by position computing circuitry if they occur within a specific time interval or timing window. This procedure results in non-productive use of the processing circuitry in computing the position of events later determined to be invalid.
Additionally, the prior art rejects a significant number of valid events because of the phenomenon known as “pile-up.” Pile-up occurs when two events occur so close together in time that their amplitudes are erroneously combined in the detector. Such “piled-up” events typically are rejected by prior art detectors because they are detected as a single event with an energy level that exceeds the predetermined maximum energy threshold. Thus, there remains a need in the art to improve upon the throughput speed and accuracy of acquired images in a positron coincidence imaging system.